Recently, W-CDMA (Wideband-Code Division Multiple Access, see Non-Patent Document 1) defined as RAT (Radio Access Technology) that is radio access technology by 3GPP (3rd Generation Partnership Project) has been standardized as a third generation cellular mobile communication system, and services thereof have sequentially been provided. Additionally, EUTRA (Evolved Universal Terrestrial Radio Access) and EUTRAN (Evolved Universal Terrestrial Radio Access Network) have been under consideration. In EUTRA, OFDMA (Orthogonal Frequency Division Multiple Access) has been proposed as a communication scheme (see Non-Patent Document 2).
In cellular mobile communication systems, mobile station devices included in a cell or a sector, which is a communication area served by a base station device, have to be wirelessly synchronized with the base station device in advance. For this reason, the base station device transmits SCH (Synchronization Channel) having a given structure so that the base station device detects the SCH to be synchronized with the base station. In W-CDMA, P-SCH (Primary SCH) and S-SCH (Secondary SCH) are transmitted as SCH in the same timing. The mobile station device achieves slot synchronization by P-SCH and frame synchronization by a transmission pattern of S-SCH, and specifies a cell ID group for identifying the base station device. Further, CPICH (Common Pilot Channel) is used to identify a cell ID of the base station device from the cell ID group (see “2-2-2. Cell Search” on pages 35-45 of Non-Patent Document 1).
A series of the above control, i.e., control by the mobile station device wirelessly synchronizing the base station device and then specifying the cell ID of the base station device, is called cell search. The cell search can be classified into initial cell search and neighboring cell search. The initial cell search is cell search for a mobile station device to search the nearest cell to stay therein after the mobile station device is powered on. The neighboring cell search is cell search for the base station device to search a candidate cell to be a handover destination after the initial cell search.
Although EUTRA is multi-carrier communication using OFDMA and therefore uses SCH, it is known that control different from cell search in W-CDMA is necessary. For example, base station devices having different band widths (for example, 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz, and 20 MHz) have to be supported for coexistence between 2G and 3G services in EUTRA. In consideration of this, a method of performing transmission in the center of the frequency bandwidth of the base station where a frequency bandwidth of SCH is 1.25 MHz has been proposed (see Non-Patent Document 2). FIG. 20 illustrates the relationship between SCH and different frequency bands B1 to B5 of base station devices. In other words, it shows the case where base station devices have frequency bands B1=20 MHz, B2=10 MHz, B3=5 MHz, B4=2.5 MHz, and B5=1.25 MHz, and SCH is allocated in the center of each of the frequency bands of the base station devices. As shown in FIG. 20, SCH is always transmitted in the center of each of the frequency bands of the base station devices even if the base station devices have different frequency bandwidths. Accordingly, even if a neighboring base station device has a different frequency bandwidth from that of the current base station device, the mobile station device can detect SCH by performing neighboring cell search in the center of the frequency band.
An SCH structure using GCL (Generalized Chirp Like) sequence has been proposed in 3GPP (see Non-Patent Document 3). According to Non-Patent Document 4, GCL sequence sk is defined as Expression (1).Sk=akb(k)mod m, k=0, . . . , N−1  (1)
where “(k) mod m” denotes a remainder when k is divided by m. Further, ak can be expressed as Expression (2).
                              a          k                =                  {                                                                      W                  N                                                                                    k                        2                                            /                      2                                        +                    qk                                                                                                (                                      IF                    ⁢                                                                                  ⁢                    N                    ⁢                                                                                  ⁢                    IS                    ⁢                                                                                  ⁢                    EVEN                                    )                                                                                                      W                  N                                                                                    k                        ⁡                                                  (                                                      k                            +                            1                                                    )                                                                    /                      2                                        +                    qk                                                                                                (                                      IF                    ⁢                                                                                  ⁢                    N                    ⁢                                                                                  ⁢                    IS                    ⁢                                                                                  ⁢                    ODD                                    )                                                                                        (        2        )            
where q is an integer and Wn=exp(−j2πr/n) where j is an imaginary unit, r is a GCL sequence index and an integer coprime to n.
bi (i=0, . . . , m−1) is a complex number whose amplitude (absolute value) is 1. GCL sequence disclosed in Non-Patent Document 3 is GCL sequence sk when bi=1, q=0, and N is a prime number, which is expressed by Expression (3).
                              s          k                =                  exp          ⁡                      (                                          -                2                            ⁢              π              ⁢                                                          ⁢              u              ⁢                                                          ⁢                                                k                  ⁡                                      (                                          k                      +                      1                                        )                                                                    2                  ⁢                  N                                                      )                                              (        3        )            
where k=0, . . . , N−1, u=1, . . . , N−1, and u denotes a GCL sequence index (corresponding to r in Expression (2)). A value of the GCL sequence index u corresponds to unique cell information and can indicate unique cell information. The unique cell information includes a cell or sector index (cell/sector ID, or cell/sector number), the number of transmission antennas included in a cell or a sector, the length of a GI (Guard Interval), a frequency bandwidth of BCH (Broadcast Channel), timing of a first wireless frame, system parameters of a cell or a sector, and the like.
Hereinafter, a cell search method disclosed in Non-Patent Document 3 is explained. FIG. 21 is a schematic block diagram illustrating the configuration of an SCH transmitter 1000 included in a base station device. As shown in FIG. 21, the SCH transmitter 1000 includes: a GCL sequence generator 1001 that generates GCL sequence having u as a GCL sequence index based on the unique cell information u; a mapper 1002 that maps the generated GCL sequence to a frequency axis; an IDFT (Inverse Discrete Fourier Transform) unit 1003 that performs an inverse Fourier transform to convert the mapped signal into a time domain signal; a GI adder 1004 that adds a guard interval to the converted time domain signal; a DAC (Digital to Analogue Converter) 1005 that converts the digital signal with the GI added into an analog signal; and a radio unit (TX) 1006 that transmits the analog signal on a carrier of a given frequency through an antenna 1007.
The GCL sequence generator 1001 generates GCL sequence s based on Expression (3) and the unique cell information u. The mapper 1002 maps respective elements s0, . . . , sN-1 of the generated GCL sequence s onto subcarriers on the frequency axis. At this time, respective elements of the GCL sequence s are mapped onto even-numbered subcarriers (subcarrier 2, subcarrier 4, . . . , subcarrier 2N) as shown in FIG. 22, null signals (signals having the power level 0) are mapped onto subcarrier 0 and odd-numbered subcarriers. Thereby, the time domain signal subjected to the IDFT by the IDFT unit 1003 becomes a repetition of the same signal.
FIG. 23 is a schematic block diagram illustrating the configuration of a cell search unit included in the mobile station device. As shown in FIG. 23, the cell search unit includes: a radio unit (RX) 1101 that receives a signal transmitted from the base station device through an antenna 1110; an ADC (Analogue to Digital Converter) 1102 that converts the analog signal received by the radio unit into a digital signal; an SCH symbol timing detector 1103 that detects SCH symbol timing using a time domain signal converted into the digital signal; a DFT (Discrete Fourier Transform) unit 1104 that performs a Fourier transform to convert the time domain signal output from the ADC 1102 into a frequency domain signal based on information concerning the SCH symbol timing detected by the SCH symbol timing detector 1103; a GCL sequence acquirer 1105 that acquires GCL sequence s′ from the signal subjected to the DFT; a differential encoder 1106 that performs differential coding on information concerning phase of the acquired GCL sequence s′; an IDFT unit 1107 that performs an inverse Fourier transform on the signal encoded by the differential encoder 1106; a peak power calculator 1108 that calculates the peak power level of the signal output from the IDFT unit 1107; and a unique cell information estimator 1109 that estimates a GCL sequence index u of the GCL sequence s′ based on the IDFT index number corresponding to the peak power level and outputs a value of the estimated u as unique cell information.
As shown in FIG. 22, the time domain SCH signal is a repetition of the same signal as explained above. The SCH symbol timing detector 1103 estimates SCH symbol timing by detecting the peak of correlation values between a reception signal and a reception signal delayed by a half symbol. The DFT unit 1104 performs a Fourier transform in the estimated symbol timing to acquire a frequency domain signal of SCH. The GCL sequence acquirer 1105 can acquire GCL sequence s′ by extracting signals of even-numbered subcarriers. The differential encoder 1106 performs differential coding on information concerning the phase of the GCL sequence s′. In other words, the differential encoder 1106 outputs a sequence including elements obtained by subtracting anterior elements s′k-1 from elements s′k each included in the GCL sequence s′.
The IDFT unit 1107 performs an N-point inverse Fourier transform on the sequence including N elements output from the differential encoder 1106 to generate a time domain signal. The peak power calculator 1108 calculates power levels of the signals which correspond to the indexes 0 to N−1 and are generated by the IDFT unit 1107. The unique cell information estimator 1109 detects, from the calculated power levels, an impulse (peak power level) corresponding only to an index u uniquely defined by the GCL sequence index u of the GCL sequence s′, and outputs the index u as unique cell information u. However, an impulse does not occur due to the effect of noises or signals transmitted from other base stations. Therefore, the unique cell information estimator 1109 estimates unique cell information u by detecting the index corresponding to the maximum power level among power levels of indexes.
Thus, wireless synchronization and identification of the cell ID (estimation of unique cell information u) can be achieved.
[Non-Patent Document 1] Keiji Tachikawa, “W-CDMA mobile communication system”, ISBN4-621-04894-5, first published on Jun. 25, 2002 by Maruzen Co., Ltd.
[Non-Patent Document 2] 3GPP TR (Technical Report) 25.814, V1.5.0 (2006-5), “Physical Layer Aspects for Evolved UTRA”, [online], <URL: http://www.3 gpp.org/ftp/Specs/html-info/25814.htm>
[Non-Patent Document 3] R1-051329 “Cell Search and Initial Acquisition for OFDM Downlink” 3GPP TSG RAN WG1 #43 on LTE Seoul, Korea, Nov. 7-11, 2005, [online], <URL: http://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1—43/Docs/R1-051329.zip>
[Non-Patent Document 4] B. M. Popovic, “Generalized Chirp-like Polyphase Sequences with Optimal Correlation Properties”, IEEE Trans. Info. Theory, vol. 38, pp. 1406-1409, July 1992